The present invention relates to the field of conveyor belts which are reinforced with a textile ply.
Known conveyor belts are principally divided into two groups: conveyor belts without textile reinforcement and conveyor belts that are reinforced at least with a piece of flat textile material.
Conveyor belts without textile reinforcement consist of a homogenous, relatively thick sheet of a thermoplastic plastic. This plastic must meet the required surface properties as well as also transmit the tractive forces acting in the belt. In a special version, a flat textile material is laminated on the running side of the belt. This helps with transmitting the tractive forces. Conveyor belts without textile reinforcement on the one hand have the advantage that they are able to be cleaned well thanks to the entirely pore free surface. Neither can lint or loose fibres lead to contamination. These advantages are however no longer applicable as soon as a flat textile material is applied on the running side. The method for implementing end-to-end joints is very simple and demands only a few technical tools. On the other hand they have the disadvantage that with increasing length of service they elongate and therefore have to be shortened again and again and joined end-to-end anew. This behaviour arises through the creeping of the thermoplastic under the constant tensile stress. Frequently an insufficient flatness of the belts is observed, in particular in the region of the end-to-end joint.
In the case of conveyor belts without textile reinforcement, to produce the end-to-end joint both ends are cut perpendicularly to the direction of travel, or at a slight angle from perpendicular, e.g. of 75°. The ends are abutted and coalesced together with heat and pressure or welded together with a filler filament of thermoplastic plastic.
Conveyor belts with reinforcement with a flat textile material comprise at least a fabric ply, which can be arranged on the running side, in the inside or on the conveying side of the conveyor belt. The ply or the plies of fabric also have the task to absorb the forces that the conveyor belt has to transmit. The reinforced fabric plies are coated on one side or both sides with thermoplastic plastic. These coatings cater for the required surface properties (friction factor, abrasion resistance) and produce a closed, easy to clean surface. Conveyor belts with textile reinforcements also have the advantage that despite the constant tensile stress that they are exposed to, they elongate only very slightly. They are characterised by a good flatness, also in the region of the end-to-end joint. It enables thin, and correspondingly flexible conveyor belts with high resistance to tearing, to be made. The fabric reinforcements on the conveying side or the running side have disadvantageous effects on the cleaning properties: the pores in the fabric which arise after some period of service, fill up with contaminants, which can only be removed with difficulty. In the area of foodstuffs conveyance this frequently forms a medium for undesired or dangerous microorganisms and fungi. If the belt is damaged by mechanical influences on the edge or on the backside, the reinforcement fabric layers begin to wear slightly or to fray. Fibres or fibre residues that have worked their way out of the bonded material may contaminate the conveyed good, and may compromise the function of the belt.
In order to make fabric reinforced conveyor belts endless, so called finger end-to-end joints are made: The two ends of the belt are cut out zigzag shaped, the zigzags pushed into each other, and the thermoplastic layers are welded by means of temperature and pressure. The points of the zigzags are thereby formed so that their flanks typically are at an angle, measured in the clockwise direction or anti-clockwise direction, of 170° to 175° to the middle line of the conveyor belt. It is generally assumed in the art that finger end-to-end joints are required in fabric reinforced conveyor belts. Firstly the tensile load decreases per unit length of the end-to-end joint, since the zigzags with their steep flanks increase the effective total length of the end-to-end joint. They should secondly lead to interlocking of the threads of the flat woven textile material and thereby bring about only a small decrease of the tensile strength at the end-to-end joint. (See e.g. EP-A-0 240 861, page 4, lines 31-33). Thirdly making the end-to-end joint as a finger end-to-end joint impedes the fulling or milling parallel to the end-to-end joint when the belt bends around rollers, and transforms it into a fulling or milling nearly perpendicular to the flanks of the zigzags of the end-to-end joint, which slows down the material fatigue in the end-to-end joint. According to the knowledge of the applicant, all manufacturers of fabric reinforced conveyor belts, process belts or transport belts still today use the finger end-to-end joint method to produce the end-to-end joint. The method of the finger end-to-end joint requires however relatively elaborate and expensive tools.
The object of the present invention is to develop a conveyor belt, which can be made endless by simple means. The conveyor belt should not elongate in operation, so that it neither has to be retightened nor shortened, and it should lie flat when tightened or untightened, having no corrugations or warping.